(1 )Field of the Invention
The present invention relates to a battery that is manufactured using energy beams, and to a manufacturing method of the battery.
(2) Description of the Related Art
Energy beam welding is widely used for manufacturing batteries housed in metal outer package cans, as disclosed in Japanese Laid-Open Patent Applications H4-162351, H8-293299, and 2000-77040 and Japanese Patent Publication No. H2-15985.
A cylinder alkaline storage battery, for example, is manufactured as follows. First, a bundle of electrodes and separator is made by winding a positive-electrode plate and a negative-electrode plate together with a separator between them. The positive-electrode plate and negative-electrode plate of the bundle are welded to the positive- and negative-electrode power collecting bodies, respectively. Then the bundle is housed in a metal outer package can. The positive-electrode power collecting body is connected to an opening closure member that is at an opening end of the outer package can, and the negative-electrode power collecting body is welded to the bottom of the outer package can. More specifically, the negative-electrode power collecting body is welded to the bottom of the outer package can at approximately its center by applying energy beams there. Laser beams are widely used as the energy beams.
Such an energy beam welding fixes the negative-electrode power collecting body to the bottom of the outer package can with certainty. Also, with such a method in which energy beams are applied from outside, there is no need to insert the welding rod into the bundle of electrodes and separator housed in the outer package can. This also eliminates the need to provide a space for inserting the welding rod, which provides a merit of improving the energy density in the battery.
However, the welding portion formed by this energy beam welding has relatively large crystal grains and is therefore weak. This is because the welding portion is rapidly cooled at a rate of several thousands of degrees (° C.) per several milliseconds immediately after the portion is melted by the application of energy beams. For this reason, a crack may be generated in the welding portion (the weak layer), and the crack may grow to the extent that the electrolyte leaks from the outer package can. Such a problem is generated when a battery expands and shrinks by a rapid change in the internal pressure or temperature in the battery, which occurs when, for example, a manufactured battery is used in some electric tool or the like in such a severe condition that charges and discharges are repeated, each charge and discharge being caused for a short time by a large amount of current.
Some might think that the above-mentioned problem of the weak layer, which is generated by such rapid cooling, would be solved by performing, after the application of energy beams, an annealing process in which the welding portion is left out for several hours at a temperature as high as several hundreds of degrees (° C.). However, because of the structure and nature, batteries do not tolerate being left out for a long time at a temperature as high as several hundreds of degrees (° C.). As a result, such an annealing process is impossible in reality.
As described above, such a battery manufacturing method using energy beams has the above-mentioned problem. It is also expected that a similar problem will occur if the positive-electrode power collecting body is welded to the bottom of the outer package can.